Ion mobility spectrometers (IMS) consist of an ionizer coupled via a shutter grid to an ion drift region. Gas or vapor to be analyzed is introduced into a reactor or ionizer by means of a carrier gas. In the reactor, the carrier molecules are ionized, normally by .beta. irradiation from an Ni-63 radioactive source. Reactant ions are produced from the carrier gas and product ions are produced from reaction of reactant ions with the sample. Under the influence of an applied electric field, the reactant and product ions are extracted from the plasma generated near the radioactive source and are drawn to a shutter grid where they are pulsed as finite slugs of ions into the ion drift region. The ion drift region is a cylindrical cavity around which coaxial guard rings are positioned, and at the end of which a collector is positioned, to intercept and count the ions that reach it. The motion of the ion population, during the time between its entrance into the drift region and its arrival at the collector, is determined by the electric field applied across the guard rings, the temperature and pressure within the spectrometer and the nature of the ions. Spectrometers of this type are well known in the art and are described, e.g., in GB 2217103, and in U.S. Pat. No. 4,777,363. The sensitivity of this type of spectrometer is limited by two factors. Firstly, the number of ions which are generated in the reaction zone is relatively low, and only a small percentage of them pass through the shutter grid, so that the pulse of ions reaching the drift region is low. Secondly, the trajectory of the ions entering the drift region at the shutter, until they reach the collector is influenced by the electric field generated by the coaxial guard rings. Because this trajectory is not straight, some of the ions which enter the drift region at the shutter do not reach the collector, thus reducing the ion current intensity of the reading of the spectrometer. Given the limitation of the radioactive source, which generates a finite number of ions, control over the trajectory of the ions should be gained, in order to increase this reading.
Furthermore, the spread in the drift times of ions of the same type, travelling from grid to collector, is indicative of the tube resolution. The narrower the spread the higher the resolution and the ability accurately to determine the mobility and to establish the identity of the ions.
Several attempts have been made to obtain a homogeneous electric field within the drift tube or region, mainly by increasing the size of the metallic rings, and reducing the size of spacer or insulating rings. Typically, IMS devices known in the art have ratios of about 1:10 between the width of the insulating ring to the width of the conductive metallic rings.